1. Field of the Invention
The present invention relates to RF repeaters for exchanging time division multiple access (TDMA) transmit and receive signals between a mobile handset and a signal conduit, e.g. a co-axial cable or an optical fiber cable in a cable television (CATV) plant, in a time division multiple access (TDMA) telephone system.
2. Description of the Related Art
It is known to provide telephone communication to a mobile telephone handset by connecting one or more RF repeaters in the form of remote antenna drivers (RADs) through a CATV plant to a remote antenna signal processor (RASP) and a base station. The base station interfaces with a public switched telephone network and provides TDMA transmit signals through the CATV plant to the RF repeaters for transmission as radio signals to the handset. From the handset, receive signals, the timing of which is slaved by the handset to that of the transmit signals, are transmitted as radio signals to the RADs, from which they are passed through the CATV plant to the base station for conversion and connection to the public switched telephone network.
The base station supplies the transmit signals at a first predetermined frequency to the RASP, at which the transmit signals are frequency converted to a second or intermediate frequency, filtered and again frequency converted to a third frequency, which is suitable for transmission through the CATV plant to the RADs. At the RADs, the transmit signals are frequency converted to the intermediate frequency, filtered and then frequency converted back to the first frequency, at which they are transmitted as radio signals to the handset. The receive signals are similarly frequency converted in the reverse direction.
With this arrangement, there may be a significant limitation in the cable plant in that the upstream cable path, i.e. the return path from the RF repeaters to the RASP, may be limited in dynamic range to a value much less than the dynamic range of the mobile handset and the base station. For example, the dynamic range of the latter may be in the order of 70 dB, but the range of the upstream cable path is often less than 40 dB.
This limitation is significant in providing cell coverage, as it is often the limiting factor in a two-way communication system. The downstream cable path, i.e. the transmit signal path from the RASP to the RF repeaters, does not display the same limitation. Cell size (or coverage range) is vital as it determines the viability of system deployment, and determines how many potential subscribers to the service can be reached.
There is a second limitation of active RF repeaters in a CATV based TDMA telephone system where more than one cell is used to provide extended coverage (simulcast). This second limitation is the effect of cumulative noise figures due to each of the repeaters using the same frequency band. The greater the number of RF repeaters used, and thus the larger the coverage area, the greater the cumulative noise. This noise acts to reduce the dynamic range, and thus to reduce the coverage area.
Spatial diversity at the RF repeater can be used to offset the effect of path fading that will occur on a single receive antenna. Thus, the use of a second antenna spaced at an optimum distance from the first will provide a second or diversity channel that can be presented to the base station so that the base station can use the best of the two channels, if required. This diversity option has the effect of extending relative coverage area in the receive (upstream) direction. One method to transport this diversity channel is to frequency convert it on the upstream path to a frequency band separate from that of the main channel, and then to transport them both back to the base station through the CATV upstream path. However, this has the disadvantage of requiring twice the bandwidth to be utilized on the upstream cable plant.
A fourth problem exists when transporting a receive (upstream) signal through the CATV upstream path. This observed problem is that the noise floor of the CATV upstream path tends to change dynamically over time, and can be worse in some portions of the upstream path than in others. This change in the noise present in the transport channel can cause the relative signal-to-noise ratios at the base station to change over time.
All four limitations are further complicated by the TDMA nature of the signal, and the consequential requirement for time slotted analysis of each situation.